The use of catalysts with high activity and selectivity of the Ziegler-Natta type and, more recently, of the metallocene type has led to the widespread use on an industrial scale of processes in which the olefin polymerization is carried out in a gaseous medium in the presence of a solid catalyst. An example of said gas-phase polymerization processes involves the use of a fluidized bed reactor wherein a bed of polymer particles is maintained in a fluidized state by the upward flow of a fluidizing gas.
During the polymerization fresh polymer is generated by catalytic polymerization of the monomers and the manufactured polymer is drawn off from the reactor to maintain the polymer bed at a constant volume. The fluidized bed, which comprises a bed of growing polymer particles and catalyst particles, is maintained in a fluidization state by the continuous upward flow of a fluidizing gas, which comprises the recycled gas stream and make-up monomers. Industrial processes employ a distribution plate to dispense the fluidizing gas to the polymer bed, the distribution plate acting also as a support for the bed when the supply of gas is cut off. The fluidizing gas enters the bottom of the reactor and is passed through the distribution plate to the fluidized polymer bed.
It is known that in the gas-phase (co)polymerization of olefins, the monomers and comonomers generally remain occluded in the porous polymer particles, in particular when the comonomers are α-olefins comprising from 4 to 8 carbon atoms. For instance, in the manufacture of linear low density polyethylene (LLDPE) 1-hexene is mainly used as a comonomer, while in the manufacture of high density polyethylene (HDPE) a relatively high polymerization temperature and a large amount of hydrogen used as a molecular weight regulator may promote secondary hydrogenation reactions, in particular forming organic compounds of a low volatility containing for instance, from 4 to 12 carbon atoms.
Moreover, the gas-phase polymerization of olefins can be carried out in the presence of inert gases, such as propane, isobutane, isopentane or other saturated aliphatic hydrocarbons, which have mainly the function of contributing to dissipate the heat generated inside the reactor from the polymerization reaction. Even these alkanes of low volatility may remain occluded and dissolved in the porous polyolefins particles.
For safety, economical and ecological reasons, there is the need to remove the unreacted (co)monomers, the organic compounds and alkanes of relatively low volatility from the produced polyolefin. All these compounds constitute a load on the environment, some of them are capable of forming explosive mixtures in the presence of atmospheric oxygen. Unconverted monomers represent also a risk of prolongation of uncontrolled residual polymerization outside the polymerization reactor.
Another drawback associated with the presence of unconverted monomers, alkanes and oligomers inside the produced polyolefin is given by the development of bad smell, which hinders the marketing of the molded articles in food and pharmaceutical applications. In a particular way, the presence in the polyolefin of oligomers, saturated and unsatured hydrocarbons with more than 5 carbon atoms, such as for instance 1-hexene and hexane, is responsible of bad smell development. The content of these components has therefore to be drastically reduced especially in case the produced polyolefin is aimed to be molded for manufacturing items for food applications.
EP 808850 discloses a method of reducing the odor development in olefinic polymers obtained by means of a metallocene catalyst in a gas-phase polymerization process.
According to the teaching of this patent the ligands having a cyclopentadienyl skeleton are sources of odor development and they can be efficiently removed by a method including a step of contacting the polyolefin with a ligand decomposer, such as water or alcohol, to decompose the residual ligands contained in the polyolefin and then a step of removing the decomposed ligands by heating said polyolefin.
EP 1348721 relates to ethylene copolymerization for producing an ethylene copolymer having a low content of components that might generate odors or components that might change the taste of foods. The disclosed process comprises the gas-phase polymerization by means of a metallocene catalyst in a fluidized bed reactor with a saturated aliphatic hydrocarbon existing in the reactor in a concentration of 2 to 30% by mol. The copolymer powder withdrawn by the reactor is then subjected to a ligand-decomposing step contacting the polyolefin with water, oxygen or alcohols and then to a ligand-removing step by heating said copolymer.
U.S. Pat. No. 6,218,504 relates to a process to deodorize polyolefins and also to the use of deodorized polyolefin granules to produce plastic moldings with a low taste-impairment and odor-impairment. The process contemplates the use of a specific apparatus, wherein a gaseous mixture of steam and air, or a gaseous mixture of steam and nitrogen, or alternatively pure steam is passed around the polyolefin granules at a temperature preferably in the range from 90° C. to 130° C., with an amount of steam from 0.3 to 2.0 Kg per kg of polyolefin granules. U.S. Pat. No. 5,376,742 discloses the recovery of unreacted monomers from a polymer product coming from a fluidized bed reactor, and employing such recovered gases to purge the unreacted monomers from the polymer product. Ethylene is catalytically copolymerized with a C3-C8 olefin in a fluidized bed reactor in the presence of nitrogen as the reaction diluent. The obtained ethylene copolymer is counter-currently contacted with a gaseous stream comprising mostly ethylene and nitrogen to produce a copolymer having a reduced content of unreacted monomers. Said gaseous stream comprising mostly ethylene and nitrogen is derived from the cooling of the reaction gas mixture, which causes a partial condensation of the unreacted C3-C8 olefin comonomers and the simultaneous formation of a gaseous stream enriched in ethylene and nitrogen. The disadvantage correlated with the use of a gaseous stream comprising a major part of ethylene, as the purge gas for degassing the polymer, is the potential for further reaction of the still active polymer powder present inside the degassing vessel. If allowed to occur, this may lead to form agglomerates in the degassing vessel and a change in powder physical properties.
The disclosure of WO 03/011920 is aimed to solve the above mentioned problem by the removal of substantially all the monomers, for example ethylene, from the gaseous stream prior its use in the degassing vessel. According to the teaching of this patent the gaseous stream coming from the reactor, before its use as stripping agent in the degassing vessel, is passed to a first recovery unit for removing the heavy hydrocarbons, typically those comprising from 5 to 8 carbon atoms, which are especially used in the manufacture of LLDPE. After the removal of the heavy hydrocarbons, carried out for instance by refrigeration, the gaseous stream is passed to an ethylene recovery unity, so that the degassing stream entering the degassing vessel is advantageously enriched in nitrogen and substantially free of ethylene and comonomers, thus reducing the risk of further uncontrolled polymerization inside the degassing vessel. A disadvantage of this technique is that the gas is directly taken from the recycle line of the reaction gas to the reactor and fed back to it after the degassing. Therefore the degassing cycle is highly dependent on the conditions, particularly the pressure, in the reactor.
EP 683176 relates to a process for the continuous manufacture of ethylene (co)polymers by gas-phase catalytic copolymerization, the process leading to a decrease of the amount of undesirable volatile organic compounds in the obtained ethylene copolymers. After the transfer of the reactor effluent to a depressurization zone to achieve a raw separation of the solid phase from the gas phase, the solid phase is subjected to a multistage stripping process by means of (1) at least one non-deactivating flushing with a gas mixture which is substantially free from any poison to the active catalytic residues, and subsequently (2) a deactivating flushing with a gaseous mixture of nitrogen, water and oxygen.
The gaseous mixture used to carry out the above non-deactivating flushing (1) may be chosen from nitrogen, the gaseous reaction mixture and one or more of the constituents of said gaseous reaction mixture, preferably mixed with nitrogen. It is preferred the use of the gaseous mixture circulating in the polymerization reactor. The gaseous mixture used to carry out the deactivating flushing (2) essentially contains nitrogen and very low amounts of water or oxygen. Water is used in an amount of from 50 to 4000 ppm by weight relative to the flushed copolymer. Oxygen is used in an amount of from 5 to 1000 ppm by weight relative to the flushed copolymer. Accordingly, due to such a poor amount, water or oxygen have essentially the function of deactivating the catalytic residues in the ethylene copolymer, while nitrogen is the only stripping agent for decreasing the content of volatile organic components in the polyolefin. The process of EP 683176 is a double-stage stripping which requires the use of high amounts of nitrogen: this technique is very costly, since high amounts of nitrogen have to be heated to a high temperature to ensure the stripping efficiency. In particular, the technique described in this patent is particularly burdensome for all the gas-phase polymerization processes carried out in the absence of N2 and using a saturated aliphatic hydrocarbon as the polymerization inert.
It would be desirable to overcome the drawbacks correlated with the use of unconverted monomers and/or nitrogen in the polymer degassing, when a high efficiency of removal of oligomers and hydrocarbons from the polyolefin granules is required.
The Applicant has now found a process for decreasing considerably the odor development from polyolefin granules obtained by a gas-phase polymerization, which is operated in the presence of a saturated aliphatic hydrocarbon.